This invention relates to an improved process for the preparation of quinacridonequinone and its substituted derivatives.
Quin(2, 3b)acridine-6,7,13,14(5,12H)tetrone, more commonly called quinacridonequinone, is a well known compound which is used in the manufacture of commercial pigments. Quinacridonequinone and its substituted derivatives have been prepared by the condensation of anthranilic acid with benzoquinones under the influence of suitable oxidizing agents, followed by cyclization under the influence of such dehydrating agents as concentrated sulfuric acid, as described, for example, in J. Russ. Phys. Chem. Soc., 47, 1260-8 (1915), CA 9, 3056 (1915), and in U.S. Patent 3,185,694. Quinacridonequinones are commonly prepared commercially by the oxidation of quinacridone with sodium bichromate in acid medium.
A process for preparing quinacridonequinones directly from the corresponding dihydroquinacridones is described in U.S. Pat. No. 3,251,845. This process involves heating the dihydroquinacridone in an acid medium containing an anion from the group consisting of chromate ion, permanganate ion, and nitrate ion, with the provision that the permanganate ion may be used in an alkaline medium. Although the quinacridonequinones produced according to U.S. Pat. No. 3,251,845 are highly satisfactory for many pigmentary applications, the purity and intensity of color are not as high as desirable for especially high quality pigmentary applications. Furthermore, the oxidation reaction produces a relatively large concentration of heavy metal ions which must be removed from the reaction medium prior to disposal to avoid environmental pollution.
This invention provides for an improved process for producing highly pure and color intense quinacridonequinones from dihydroquinacridones without the need for high concentrations of heavy metal oxidants.